Many industrial processes emit dust from stacks. An example is coal-fired power stations. There is often a statutory requirement to continuously monitor the amount of dust being emitted.
A favoured technique is opacity monitoring. Here a light beam is projected across the stack and its attenuation measured. This measurement is correlated to the dust density in the stack.
Practical opacity monitors use an arrangement in which both the light source and detection are located at one side of the stack and a reflector at the other side. The advantage of using a retro-reflector assembly is based on the well-known property of a corner-cube prism to return a light beam exactly along its incident direction—irrespective of the precise orientation of the prism. This makes the system tolerant to misalignment of the retro-reflector assembly.
Opacity is derived from measurement of the projected and returned beam and is typically expressed as a percentage. If the returned beam power is say 70% of the projected beam power then the opacity is 30%.
However, opacity measurement becomes difficult in relatively clean, low-dust processes where the opacity is small—for example 5%. Consequently, if the measurement of either projected or returned beam power is subject to a drift of say 2 percent, then the error in the measured opacity also drifts 2 percent—ie two fifths of its true (5%) value.
Thus there is great interest in making opacity monitors with the highest possible stability. This stability must be maintained over time (many months) and temperature (a 40F. temperature cycle is a standard test stipulated by the USA Environmental Agency).
The most obvious sources of drift are in the light emission and detection assembly. This is a complex device often referred to a transmissometer and a good transmissometer will have a stability of about 2 percent.
However, very recent development work has resulted in transmissometer designs which are capable of stabilities of a few tenths of one percent. For example see U.S. Pat. No. 6,781,695.
However, a retro-reflector assembly is a simple, passive device but the efficiency with which it returns the incident light beam is, nevertheless, subject to variations with time and temperature. With the latest transmissometers, retro-reflector drift can become a significant component of the overall system drift.